Development in the field of radiofrequency applications, in particular in communication systems, is heading more and more in the direction of amplifier circuits having smaller dimensions in conjunction with higher power and at the same time subject to the requirement of coping with greater bandwidths of signals to be processed. In order that the space requirement of corresponding amplifier circuits or other radiofrequency circuits is kept compact, the degree of integration of the amplifier circuits is being progressively increased. For power amplifier circuits this means considerable challenges with regard both to efficiency and to linearity and circuit stability. This applies with regard to Doherty power amplifier circuits. The latter are used in a large number of fields of application and frequency ranges, for example in the field of mobile radio systems, for example 5G and MIMO systems, or else in the field of broadcasting, and also in other wireless applications such as e.g.: WLAN.
Transistors, in particular power transistors and/or radiofrequency transistors, are a constituent of many power amplifier circuits. For some types of transistors, a capacitance between a control terminal and a load terminal of the transistor can lead to an undesired feedback behavior that can result in undesired effects, for example instabilities of the power amplifier circuit during operation of the transistor.
One conventional procedure for ensuring the stability of the transistor consists in adding a stability resistance for damping oscillations that possibly occur. A transistor having a gate terminal is used as transistor in some cases. In these cases, said resistance can be connected to the gate terminal of the transistor. In this case, however, the resistance can bring about additional disadvantageous effects for the power circuit; by way of example, the gain factor of the power circuit can be reduced by the resistance and/or the resistance can lead to a non-ideal behavior of the gain as a function of the circuit frequency, which can be problematic in particular for Doherty amplifier circuits. This applies in particular to cases in which gallium nitride transistors (GAN transistors) are used, which may have strong feedback properties. As a result of large changes in a gain factor of such an amplifier circuit with frequency, an input matching network used can be mismatched in some cases, which can result in a further loss of gain and possibly of system efficiency and hampers the design of circuits for a wide frequency range.
In order to compensate for the effects described above, in some applications it is possible to use a feedback system network comprising at least one resistance, one inductance and one capacitance (RLC network) between a control terminal and a load terminal of a transistor. In this case, the additional capacitance is used for DC voltage decoupling since load terminal and control terminal are generally at different potentials. However, said additional capacitance of the feedback network can likewise lead to undesired, disadvantageous effects.